Various circuits are known to control the supply of power, voltage, and/or current to electrical devices such as light sources of the LED and incandescent type. The supply of power is controlled for various reasons, such as dimming the light output of the light source. One technique used to control dimming of these types of light sources is pulse width modulation (PWM). In implementing the PWM technique the power supply produces a pulse whose time duration (width) is controlled during a fixed period of time. This is done repetitively. The pulse width relative to the fixed time is referred to as the duty cycle. This is shown in FIG. 1 in which the repetitive fixed period is between time points A and B and the power pulse P can have a duty cycle time of any percentage of the time between these two points. When PWM is used to control the output of a light source, the rate or frequency of the fixed time period repetition is selected so that the light source is turned on and off at a rate high enough so that the thermal lag effect of the filament inside of an incandescent lamp bulb or the elements of an LED and the persistence of vision of a human integrate out so that the human eye cannot perceive the light source turn on and off or flicker.
In the control of the lighting devices using PWM, the longer the duration of the power pulse P (that is, the larger its duty cycle), the brighter the light output will be. For example, if the pulse duty cycle is 50% of the fixed time period, the source light output will be about 50% of its rated value or, conversely, dimmed by 50%. By selecting the pulse P duty cycle using PWM, the dimming of the light source can be controlled to any desired level.
While control of the light source output can be successfully accomplished using PWM, a significant disadvantage exists. This is due to the fact that repetitive production of the power pulse P is often at a frequency such that its harmonics can cause interference with the radio frequency spectrum. Accordingly, a need exists to be able to supply power to devices (such as light sources, as well as other devices) in a manner such that the output of the devices can be controlled (such as dimming of a light source output), but without having the problem of possibly producing interference with the radio frequency spectrum.